Microorganisms are present in paper and board machines every day. Their entry into the process is inevitable, because the circulating water contains biodegradable dissolved substances, temperature and pH are typically favourable for microbial life, and the machines are open processes subject to contaminants from air, water, and non-sterile raw materials. Paper and board machines can support growth of very different kinds of microorganism; but most common organisms are bacteria and fungi. A thorough review of paper machine microbiology, about the microorganisms and their impact on paper making, is presented in Chapter 6: Paper machine microbiology, pp. 181-198, by Marko Kolari, in Handbook of Papermaking chemistry 2007, Raimo Alen (Ed.), Finnish Paper Engineers' Association, Helsinki, Finland.
Microorganisms can cause a number of problems in papermaking, if countermeasures are not taken. Microbes may affect both the functioning of the chemistry of papermaking and the quality of the end product. Starch is one of the most easily digestible nutrients for microbes, but also some other additive slurries are prone to spoilage. High microbial activity, for example in a coated broke tower, can lower the pH and thus have a marked effect on wet-end chemistry. High microbial activity can also create strong odours that may be a nuisance or even a danger to personnel, and also destructive for product quality in packaging grades. Slime formation (biofilms) on the surfaces of tanks and machine frames leads to paper defects (spots and holes) or web breaks when slime lumps are sloughing off. A web break requires cleaning and re-starting, results in system down-time, lost paper production, reduced efficiency and increased costs. It is therefore desirable to both minimize bacteria in the process waters and to prevent biofilm formation on the system surfaces.
Biocide dosing to the process waters is the most common way to combat microbes. A typical biocide program is composed of individual treatments of the different parts of the process system, such as incoming raw water, additive storages, paper machine water circuits, and the broke system. If biocide treatment in one part fails, it typically affects the whole machine. The type of biocide to be selected depends on the point of application, the performance target and chemical compatibility to the system. In the treatment of paper machine wet-end, i.e. circulating water and broke system, oxidizing biocides are nowadays widely used due to their cost-efficiency. Common oxidizing biocides are for example the “free halogen” sources such as sodium hypochlorite, hypobromous acid and chlorine dioxide. Also widely used are so called “stabilized halogens” such as halogenated hydantoins, e.g., bromochloro-dimethylhydantoin or partially halogenated hydantoins such as monochloro-dimethylhydantoin. Other common “stabilized halogen” oxidizing biocides are haloamines, such as chloramines and bromamines, formed by combining an ammonium source, such as ammonium sulfate, ammonium chloride, ammonium bromide, ammonium phosphate, ammonium nitrate or any other ammonium salt, including urea, with an oxidant such as sodium hypochlorite.
The possibility to prevent microbial problems in a cost-effective manner has been the driving force making the oxidizing biocides as the preferred biocide type in most of the paper machines. However, now the longer-term use of some of the oxidizing biocides has created a new type of problem, namely vapor phase corrosion of machinery. The different oxidizing biocide chemistries do differ in terms of volatility and life-time in the paper machine process water. In case of using extensive dosages of an oxidizing biocide type that cycles-up in the process water, and is also very volatile, this can lead to vapor phase corrosion problems. Most volatile oxidizing biocides are chlorine dioxide and haloamines. In worse cases, the corrosion of paper machinery in the beginning of drying section has created huge runnability problems and deteriorated the paper quality due to flakes of corrosion products detaching from the damaged parts of the machinery.
Corrosion is a particular concern in the “short loop,” or short circulation section, of a paper machine, and in the subsequent press and drying section. In a typical pulp and paper process, pulp stock is passed into a headbox, which distributes the pulp stock onto a moving wire in a forming section. The paper sheet is formed in the forming section and then sent to presses and dryers for finishing. The short loop is a system that re-circulates and recycles excess water from the pulp stock. The excess water is collected in a wire pit in the forming section and then a major portion thereof is recirculated back to the headbox for re-use. Although many tanks, lines and other immersed structures of pulp and paper systems are typically formed from acid-proof stainless steel, many components above the water surface level, and in the press and dryer section, are formed from milder steel materials. So these components can be adversely affected by vapor phase corrosion taking place due to intense use of volatile type of oxidizing biocides for microbe control.
The current practice of how paper makers are following vapor phase corrosion (if monitoring at all) is based on installing steel coupons (e.g., carbon steel coupons) hanging in the vicinity of the paper machine. These coupons have been accurately weighted before installation and after removal they are thoroughly washed to remove any loose corrosion products, dried and weighted again. The corrosion rate is calculated based on metal loss (weight loss) during the exposure time. Monitoring with corrosion coupons only provides delayed information and does not allow a fast reaction time. Thus, there is a need to try to overcome at least these problems.